The present invention relates to a method and an apparatus for inspecting foreign substances, such as dust, human hairs, human skin, metallic elements, glass, chipping, process products, etc., which are adhered to front faces of light-permeable bodies to be inspected, such as for example, glass plates used in liquid crystal panels, base boards of optical disks and hard disks, CCD sealing plates, surface acoustic wave (SAW) filters of electronic parts, etc. More specifically, the present invention relates to a method and an apparatus whereby only foreign substances adhering to front faces of the inspection bodies are distinguished and detected from those adhering to rear faces of the inspection bodies.
A liquid crystal panel has fine liquid crystal pixel electrodes and thin film transistors formed on a front face of a glass plate. The quality of the liquid crystal panel as a product is greatly adversely influenced if a foreign substance adheres to the front face of the glass plate. Inspection by an inspecting apparatus is thus necessary to inspect foreign substances adhering to the front face. Meanwhile, the glass substrate used in the liquid crystal panel is an almost transparent thin plate having a thickness of approximately 1 mm and a high light-permeability, which potentially leads to a problem in that not only will foreign substance adhering to the front face (referred to as a "front foreign substance"), be detected but the foreign substance adhering to a rear face (referred to as a "rear foreign substance") of the substrate will also be detected. The glass substrate is assembled into the liquid crystal panel generally in a state with the rear face fixed at the side to observe displayed images or to pass a back light. The rear face is accordingly enough to be a simple glass face and the adhesion of the foreign substance to the rear face does not matter at all so long as the adhering substance is very small. It is so adapted in many cases as not to recognize the rear foreign substance as a defect. If the rear foreign substance was detected to be a defect, even a good liquid crystal panel would be determined to be defective, thereby lowering a yield and causing a large loss of products.
In the meantime, the glass plates are recently required to be light-weight in accordance with an increasing need for light-weight liquid crystal panels, and consequently the thickness of the glass substrates is decreased from approximately from 1 mm to 0.7 mm.
Under the circumstances, the above-discussed wrong detection of the rear foreign substance draws more and more attention.
In order to detect only the front foreign substance of a light-permeable body to be inspected, e.g., glass substrate of the liquid crystal panel without detecting the rear foreign substance, various methods are carried out:
1. Light projected from each of two light sources are irradiated to the body to be inspected, resultant scattering light is detected by corresponding photodetectors, and intensities of the scattering light is compared, thereby to detect the foreign substance (Unexamined Japanese Laid-Open Patent Publication No. 63-186132).
2. A plurality of photodetecting means are arranged. Light is cast from each direction to the body to be detected and scattering light on the body is compared at the detectors, thereby to detect the foreign substance (Unexamined Japanese Laid-Open Patent Publication No. 63-241343).
The constitution in the above methods is complicated due to a requirement of installing a plurality of light sources and photodetectors.
On the other hand, there is a method of a simple constitution as follows.
3. An angle held between an angle of incidence and a photodetecting angle to the body to be inspected is set to be 90.degree. , and a light slit plate is arranged immediately before a photodetector. Only a scattering light generated by the foreign substance is guided to the photodetector, thereby to detect the foreign substance (Unexamined Japanese Laid-Open Patent Publication No. 3-72248). A technique of the method will be described with reference to FIG. 6, specifically with only the use of optical paths for the sake of brevity. A laser illumination A is generated from a laser 24 assuming an angle of incidence of 2.degree. to a glass 22 which is a body to be inspected, thereby forming an image focused on a front face of the glass 22. The front face of the glass 22 is thus illuminated by the laser illumination of a reduced size. A photodetecting angle of a photodetector 26 consisting of a photomultiplier is set to be 88.degree. so that an angle held between the photodetecting angle and the incidence of angle of the laser illumination is 90.degree.. A slit 25 is placed in front of the photodetector 26. The slit 25 is formed to correspond to an image formation position on an optical path 31a of a reflecting light or scattering light generated from a foreign substance 23a adhering to a front face part of the glass 22. In the constitution, the reflecting light or scattering light generated from the foreign substance 23a adhering to the front face of the glass 22 reaches the photodetector 26 through the optical path 31a and it is detected as the foreign substance. In contrast, a reflecting light or scattering light generated from a foreign substance 23b adhering to a rear face of the glass 22 illuminated by the laser illumination through the glass 22 runs an optical path 31b and is shut by the slit 25. Therefore, the reflecting light or scattering light from the rear foreign substance 23b does not reach the photodetector 26 and is not detected as the foreign substance.
It is difficult, however, in the above method to distinguish the front foreign substance 23a from the rear one 23b. In other words, the rear foreign substance is practically sometimes detected in the method. The reason for this will be described with reference to FIG. 7. The laser illumination A in FIG. 6 is illustrated ideally to trace a single optical path, but is actually composed of beams of a width W. Since the laser illumination A generated from the laser 24 is reduced to be focused on the front face of the glass 22, the beams of the laser illumination A can be regarded to be parallel in the vicinity of the glass 22. The parallel beams in the vicinity of the glass 22 have the width W in a vertical direction. A lower end of the beams touches the front face of the inspection substrate 22 along a route P.sub.1 Q.sub.1, and reaches the rear face in a route Q.sub.1 R.sub.1 after being refracted at the front face. An upper end of the beams comes to the front face of the substrate 22 in a route P.sub.2 Q.sub.2 and is then refracted to reach the rear face in a route Q.sub.2 R.sub.2. If a foreign substance is present in a range R.sub.1 -R.sub.2 of the rear face of the substrate 22, the beams hit the foreign substance thereby to be scattered or reflected. On the other hand, considering a route for the scattering or reflecting light from the foreign substance present at the rear face of the substrate 22 to enter the photodetector 26 through the slit 25, a part of the light generated from the foreign substance at a position S in FIG. 7 comes to the front face of the substrate 22 along a route SQ.sub.0, is refracted and enters the photodetector 26 through the slit 25. Therefore, as is clear from FIG. 7, in the event that the beam width W is larger than a predetermined value and the beams fall in the range R.sub.1 -R.sub.2 of the rear face when the foreign substance exists at a specific position in the range (in this case, position S), the illumination light is scattered or reflected, and a part of the light passes through the slit 25 to reach the photodetector 26. The foreign substance is eventually detected although the foreign substance is at the rear face of the substrate 22.
As is discussed hereinabove, conventionally, a plurality of photodetectors or the like arrangement is required to detect the foreign substance adhering to the front face of the light-permeable body to be inspected, resulting in the complicated constitution, while the simple constitution cannot fully distinguish foreign substances at the front and rear faces of the body to be inspected.